Dynamics of Local Vibrational Modes in Semiconductors

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Technical: The objective of this project is to elucidate the coupling of local vibrational modes (LVMs) of semiconductors to the phonon bath, and to the electronic band structure. The ability to directly probe these pathways and rates allows tests of theory at new levels of precision. Hydrogen embedded in semiconductors is of particular interest, since the associated LVMs can be excited cleanly, and are usually well separated in energy from the phonon bath. Materials systems consisting of hydrogen isotopes in semiconductors may be strongly influenced by the nature of this energy pathway, as the relaxation time can determine the stability of the semiconductor/hydrogen system. Previously, we studied vibrational lifetimes of various H, D and O related point defects in bulk Si and Ge using ultrafast lasers. These studies indicate extraordinary variations of a factor of 100 in vibrational decay rates. Such large variations are unusual in solids and motivated a deeper understanding. These lifetime measurements give an indication of fundamental parameters controlling Si MOSFET reliability values. The aim of this project is to explore the large structural dependence of dynamical parameters, and to obtain a better understanding of preferential coupling between local modes, the phonon bath, and the electronic system. Although the energy of the LVM is small compared to the band-gap of an intrinsic semiconductor, both p- or n-type semiconductors and laser-induced free-carrier concentrations provide energetically accessible channels. The existence of electronic pathways influences population lifetimes and linewidths and may account for the dramatic site dependence. These alternative decay channels will be studied experimentally by incorporating dopant structures as well as laser-induced e-h excitation in intrinsic materials, including Si, Ge, GaX (X = As, N, P) and ZnO. Non-Technical: This project provides integrated education and research training to graduate and undergraduate students in an interdisciplinary field including modern optics, materials science and computational modeling. Additionally, understanding excited state lifetimes on a picosecond timescale will delineate vibrational dynamics of defects and impurities in crystalline semiconductors of interest to physicists, materials scientists, chemists, and engineers. An important goal of the project is to demonstrate the efficacy of this method to a much larger community of potentially interested investigators. Also, the combination of experimental capabilities developed and supported by this research at the nearby Thomas Jefferson Laboratory and at the College of William and Mary will continue to be made available to domestic and international researchers, leveraging the impact of this project.